The present invention relates to a method for treating sludges containing biodegradable organic solids, and more particularly relates to a processing technique for aerobically digesting waste sludges, and particularly waste-activated sewage sludges, to produce an end product with a substantially reduced volatile solids content.
The present invention is based on the subject matter of a dissertation entitled "Improved Aerobic Digestion through Temperature and Solids Control" by D. A. Rein, University of Colorado, October 1974, expressly incorporated herein by reference.
Sludges accumulated in waste water treatment are broadly classified as raw (primary) sludge, chemical sludge, and biological sludge. Raw sludge is the semi-liquid slurry by-product derived from the physically separated solids from waste water. The major portion of the solids present in raw sludge is organic in nature. Chemical sludge is the slurry by-product derived from physical separation processes, such as sedimentation, filtration or flotation, that have been enhanced by the addition of a composition of matter that facilitates the solid-liquid separation. The characteristics of chemical sludges depend upon the separation enhancing composition used. The major proportion of chemical sludges generally comprises inorganic precipitates. A biological sludge is the slurry by-product derived from treatment processes in which dissolved organic material is biologically assimilated. The predominant portion of the solids derived from biological assimilation processes is organic in nature. Among the most difficult and expensive of the foregoing sludges to treat is the biological sludge, especially those produced by an activated sludge process used in conventional waste water treatment facilities. Such biological sludges, broadly referred to as secondary sludges, have a relatively high water content, poor dewatering properties, and are highly putrescible.
Many secondary sludge treatment and disposal processes are disclosed in the literature. The broad goal of all these processes is the economical alteration of the sludge characteristics so that the sludge can be returned to the environment in a safe, economical and beneficial manner. Objectives included within this primary goal, which are common to most of the prior art treatment and disposal processes, are to reduce the sludge volume by removal of a portion of the liquid fraction, to destroy the pathogens present in the sludge, to produce a useful by-product, and to stabilize or decompose the organic fraction in the sludge.
Anaerobic digestion has traditionally been used for sludge stabilization. During anaerobic digestion the organic material present in the sludge is partially oxidized to by-products such as organic acids, ammonia, methane and hydrogen sulphide. The major justification for anaerobic digestion is that it does partially decompose the organic matter in the sludge and render the sludge more acceptable for final disposal. Although the anaerobic digestion process is reasonably well understood, it still presents many operating difficulties and has a relatively high cost of operation. Moreover, a waste-activated sewage sludge is particularly difficult to stabilize by anaerobic digestion.
Aerobic digestion, an alternative biological stabilization process, has an inherent advantage over anaerobic digestion. Aerobic digestion more efficiently consumes the substrate materials in the sludge and consequently produces a more highly oxidized end product. In addition, aerobic digestion requires less supervision of and less capital expenditure in processing equipment than does anaerobic digestion, making the aerobic digestion process particularly attractive for small and modified waste water treatment plants. Aerobic digestion also produces a highly nitrified sludge, which makes the process an ideal pretreatment for sludge slated for land disposal.
Aerobic digestion is a complex biological process influenced by environmental factors such as temperature, solids concentration, oxygen concentration and pH. These factors affect not only the rate of metabolism of a given group of microorganisms, but also the type of microorganisms that develop and accomplish the decomposition of the organic matter. The type of microorganisms that are developed during the process determine both the operating characteristics of the process and the physical characteristics of the end products.